vTaskGetRunTimeStats() for CPU usage in FreeRTOS

Introduction

This function helps you see how much CPU time each task uses. It shows which tasks are working hard and which are idle.

You want to check which tasks use the most CPU in your FreeRTOS system.

You need to find tasks that slow down your program by using too much CPU.

You want to optimize your system by balancing CPU usage among tasks.

You are debugging and want to see if any task is stuck or hogging the CPU.

Syntax

FreeRTOS

void vTaskGetRunTimeStats( char *pcWriteBuffer );

You must provide a buffer (string) where the stats will be stored.

The buffer should be large enough to hold all the stats text.

Examples

This example creates a buffer, gets the CPU usage stats, and prints them.

FreeRTOS

char buffer[512];
vTaskGetRunTimeStats(buffer);
printf("%s", buffer);

Here, the stats are stored in a smaller buffer for later use.

FreeRTOS

char stats[256];
vTaskGetRunTimeStats(stats);
// Use stats string to display on screen or log

Sample Program

This program creates two simple tasks and a stats task. The stats task prints CPU usage stats every second. The FreeRTOS scheduler runs indefinitely.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include <stdio.h>

void vTask1(void *pvParameters) {
    for (;;) {
        // Task code here
        vTaskDelay(pdMS_TO_TICKS(100));
    }
}

void vTask2(void *pvParameters) {
    for (;;) {
        // Task code here
        vTaskDelay(pdMS_TO_TICKS(200));
    }
}

void vStatsTask(void *pvParameters) {
    char statsBuffer[512];
    for (;;) {
        vTaskGetRunTimeStats(statsBuffer);
        printf("CPU Usage Stats:\n%s", statsBuffer);
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

int main(void) {
    xTaskCreate(vTask1, "Task1", 1000, NULL, 1, NULL);
    xTaskCreate(vTask2, "Task2", 1000, NULL, 1, NULL);
    xTaskCreate(vStatsTask, "Stats", 1000, NULL, 1, NULL);

    vTaskStartScheduler();

    for(;;) {}

    return 0;
}

Important Notes

You must enable configGENERATE_RUN_TIME_STATS in FreeRTOSConfig.h for this function to work.

You need a timer or counter to measure run time; FreeRTOS uses this to calculate CPU usage.

The output format is a table showing each task's name, total run time, and percentage of CPU time.

Summary

vTaskGetRunTimeStats() shows CPU time used by each FreeRTOS task.

Use it to find which tasks use most CPU and optimize your system.

Remember to enable run time stats and provide a big enough buffer for results.

Practice

1. What is the primary purpose of the vTaskGetRunTimeStats() function in FreeRTOS?

easy

A. To suspend all running tasks temporarily

B. To create a new task with specific priority

C. To delete a task from the scheduler

D. To display the CPU usage time of each task

Solution

1. Step 1: Understand the function's role

   vTaskGetRunTimeStats() is designed to collect and display CPU usage statistics for each task in FreeRTOS.

2. Step 2: Compare options with function purpose

   Options A, B, and C relate to task management but not CPU usage reporting, which is the key role of vTaskGetRunTimeStats().

3. Final Answer:

   To display the CPU usage time of each task -> Option D

4. Quick Check:

   Function purpose = CPU usage display [OK]

Hint: Remember: vTaskGetRunTimeStats shows CPU time per task [OK]

Common Mistakes:

• Confusing it with task creation or deletion functions
• Thinking it suspends tasks instead of reporting stats
• Assuming it modifies task priorities

2. Which of the following is the correct way to call vTaskGetRunTimeStats() to store the stats in a buffer named buffer?

easy

A. vTaskGetRunTimeStats(buffer);

B. vTaskGetRunTimeStats(&buffer);

C. vTaskGetRunTimeStats(buffer[]);

D. vTaskGetRunTimeStats(*buffer);

Solution

1. Step 1: Check function parameter type

   vTaskGetRunTimeStats() expects a pointer to a char array (string buffer), so passing the buffer name directly is correct.

2. Step 2: Analyze each option's syntax

   vTaskGetRunTimeStats(buffer); passes the buffer correctly. vTaskGetRunTimeStats(&buffer); passes address of buffer pointer (wrong). vTaskGetRunTimeStats(buffer[]); uses invalid syntax. vTaskGetRunTimeStats(*buffer); dereferences buffer incorrectly.

3. Final Answer:

   vTaskGetRunTimeStats(buffer); -> Option A

4. Quick Check:

   Pass buffer name directly [OK]

Hint: Pass buffer name directly, no & or * needed [OK]

Common Mistakes:

• Using &buffer which is wrong for arrays
• Adding [] inside function call
• Dereferencing buffer pointer incorrectly

3. Given the following code snippet, what will be the output stored in statsBuffer?

char statsBuffer[100];
vTaskGetRunTimeStats(statsBuffer);
printf("%s", statsBuffer);

Assuming two tasks "TaskA" and "TaskB" run and TaskA used 70% CPU, TaskB used 30%.

medium

A. TaskA 7
TaskB 3

B. TaskA 70%
TaskB 30%

C. TaskA 7000
TaskB 3000

D. TaskA 700
TaskB 300

Solution

1. Step 1: Understand output format of vTaskGetRunTimeStats()

   The function outputs CPU usage as run time counter ticks, not percentages. If TaskA used 70% CPU, it might show 7000 ticks and TaskB 3000 ticks (assuming 10000 total ticks).

2. Step 2: Match options with expected output

   TaskA 7000 TaskB 3000 shows usage in ticks (7000 and 3000), matching expected output. Options B, C, and D show percentages or smaller numbers, which are not the direct output format.

3. Final Answer:

   TaskA 7000 TaskB 3000 -> Option C

4. Quick Check:

   Output shows ticks, not percentages [OK]

Hint: Output shows ticks, not percent values [OK]

Common Mistakes:

• Assuming output shows percentages directly
• Confusing ticks with smaller numbers
• Expecting formatted percentages in output

4. You called vTaskGetRunTimeStats(statsBuffer); but the output is empty. What is the most likely cause?

medium

A. The function requires a NULL pointer as argument

B. Run time stats collection is not enabled in FreeRTOSConfig.h

C. The buffer size is too large

D. The tasks have zero priority

Solution

1. Step 1: Identify prerequisites for run time stats

   Run time stats must be enabled in FreeRTOSConfig.h (configGENERATE_RUN_TIME_STATS set to 1) for vTaskGetRunTimeStats() to work.

2. Step 2: Analyze other options

   Buffer size being too large does not cause empty output. Passing NULL is invalid. Task priority does not affect stats collection directly.

3. Final Answer:

   Run time stats collection is not enabled in FreeRTOSConfig.h -> Option B

4. Quick Check:

   Enable run time stats in config [OK]

Hint: Check configGENERATE_RUN_TIME_STATS is enabled [OK]

Common Mistakes:

• Assuming buffer size too large causes empty output
• Passing NULL pointer mistakenly
• Thinking task priority disables stats

5. You want to monitor CPU usage of tasks over time using vTaskGetRunTimeStats(). Which approach correctly ensures accurate and meaningful stats?

hard

A. Enable run time stats, provide a large buffer, call periodically, and reset counters between calls

B. Call vTaskGetRunTimeStats() once at startup with a small buffer

C. Disable run time stats and manually count task switches

D. Call vTaskGetRunTimeStats() inside each task to get its own CPU usage

Solution

1. Step 1: Understand requirements for accurate CPU stats

   Run time stats must be enabled, buffer must be large enough to hold all task info, and stats should be collected periodically to track changes over time.

2. Step 2: Evaluate each option

   Enable run time stats, provide a large buffer, call periodically, and reset counters between calls describes the correct approach including resetting counters to avoid cumulative overflow. Call vTaskGetRunTimeStats() once at startup with a small buffer is insufficient (only once, small buffer). Disable run time stats and manually count task switches disables stats, losing automation. Call vTaskGetRunTimeStats() inside each task to get its own CPU usage is incorrect because stats are global, not per task.

3. Final Answer:

   Enable run time stats, provide a large buffer, call periodically, and reset counters between calls -> Option A

4. Quick Check:

   Enable + buffer + periodic calls + reset = accurate stats [OK]

Hint: Enable stats, use big buffer, call regularly, reset counters [OK]

Common Mistakes:

• Calling once only at startup
• Disabling run time stats
• Calling inside each task expecting per-task stats